Backup power supply system

ABSTRACT

A balancing system for balancing respective voltages of N consecutively connected capacitors during charging or discharging includes balancing units, each having a pair of associated switches and an electromagnetic coil. An inter-switch junction is connected to an inter-capacitor junction of a corresponding group of capacitors through the electromagnetic coil. A control and generation circuit generates PWM control signals and transmits a generated PWM control signal to each of the switches. The PWM control signals have fixed duty cycles that do not vary temporarily during charging or discharging of the N capacitors. The duty cycles of two PWM control signals transmitted to two associated switches are complementary for each balancing unit.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to European Patent Application No. EP19196244.8, filed on 9 Sep. 2019.

TECHNICAL FIELD

The present disclosure relates to the field of power supply systems andmore precisely to backup power supply systems. Such a system can beused, for example, in a vehicle to provide power to an on-boardelectrical circuit in case an on-board battery is momentarilyinterrupted.

BACKGROUND

In a vehicle, an electrical battery (typically a battery of 12V) isprovided on board to supply power to electrical devices of the vehicle.Under certain circumstances, the power supply from the battery may betemporary interrupted, typically during a time interval from severalmilliseconds to several seconds, due to an undesired disconnection ofthe battery. In such a situation, it is needed to supply power to theelectrical devices of the vehicle from a backup power supply system.Capacitors are usually used as energy buffers in such a backup powersupply system.

FIG. 11 shows schematically a source power, such as a battery bat havinga voltage V_bat that supplies a load P with an electric current i(t). Aplurality of serially connected capacitors C are connected in parallelto the load P and acts as an energy buffer. When the connection of theload P with the power source bat is interrupted, as representedschematically when the switch SW is open (off), the battery supply isinterrupted and the capacitors C supply current i(t) to the load P.

In a vehicle, devices such as ADAS (Advanced Driver Assistant System)controllers require such backup power supply.

In order to face a disconnection of the battery lasting up to severalseconds, it is needed to store a high amount of energy. For thispurpose, it is known to use specific capacitors, called supercapacitors(or ultracapacitors). A supercapacitor is a high-capacity capacitor witha capacitance value much higher than other capacitors, but with lowervoltage limits. The maximum charge of a supercapacitor is for examplearound 2.5 V. Charging a supercapacitor beyond its maximal charge maycause damage. Consequently, in order to provide a backup power supplywith a desired high voltage (superior to the voltage limit imposed bythe capacitor), it is necessary to connect a plurality of capacitors inseries. In a vehicle having a 12V battery, six or seven capacitorsserially connected are needed to achieve the backup power supply.

The capacitance values of manufactured capacitors having a same maximumcharge are usually very dispersed. Typically, variations of about 20% to30% can be observed.

It results from the above that, when a stack of capacitors, including Ncapacitors serially connected, is charged from a power source, it isneeded to balance the voltages of the different capacitors in order toavoid that one or more capacitors be charged above its maximum charge.

Different techniques are known to balance the voltages of the capacitorsof a stack of capacitors.

With reference to FIG. 12, a first technique uses a resistance Rhalf-bridge to balance the voltages of at least two capacitors C1, C2serially connected. The higher is the resistance value, the longer isthe charging time period. With capacitors having capacitance values ofaround the kilofarad and resistances R having resistive values around 10kΩ, the time constant of the capacitors is around megaseconds andconsequently the charging time is very long. In practice, such asolution is not interesting.

With reference to FIG. 13, a second technique, that is a variant of thefirst technique, is based on a circuit having also a resistance Rhalf-bridge connected to at least two serially connected capacitors C1,C2, wherein the inter-resistance junction is connected to theinter-capacitor junction through an amplifier R and a damping resistorr, serially connected. Such a balancing circuit allows a rather quickbalancing but produces a lot of heat.

Linear Technology Corporation designs and manufactures high currentsupercapacitor charger and backup supply, such as the productsreferenced LTC3350 and LTC3625. Such products provide an automaticbalancing of the capacitors but have two main drawbacks. These productsare expensive and the automatic balancing requires several minutes toachieve the balancing.

In the automotive environment, a maximum balancing time of a few secondsis desired.

With reference to FIG. 14, a third technique uses a switched powersupply (or switching mode regulator) with a feedback (or regulation)loop. As represented in FIG. 14, in order to charge for example twocapacitors C1, C2 serially connected, two serially connected switchesSW1, SW2 are connected in parallel to the two capacitors C1, C2 and theinter-switch junction and the inter-capacitor junction are connectedthrough an electromagnetic coil L. The two switches SW1, SW2 and the twocapacitors C1, C2 are connected to a power source, providing a voltageu_total and a current i_charge, in order to charge the capacitors C1,C2. A controller CT produces PWM control signals, PWM1 and PWM2, tocontrol the switches SW1 and SW2, respectively, in such a way that thecapacitor voltages u_C1 and u_C2 are balanced. In addition, the voltageof the capacitors is measured and transmitted to the controller CT as afeedback signal V_fb (voltage signal at the inter-capacitor junction).The controller CT modulates, or regulates, the duty cycles of the PWMcontrol signals PWM1 and PWM2 transmitted to the two switches SW1 andSW2, depending on the feedback signal, in order to balance the capacitorvoltages u_C1 and u_C2. Such a switched power supply system with afeedback loop offers a fast and efficient balancing. However, it has amajor drawback of being expensive.

It is needed to improve the situation. More precisely, it is needed toprovide a cheaper alternative backup power supply system that achieves afast and efficient balancing of serially connected capacitors.

SUMMARY

The present disclosure concerns a balancing system for balancingrespective voltages of N consecutively connected capacitors of a backuppower supply system during charging or discharging of said N capacitors,n groups of at least two consecutive capacitors, with n≤N−1, beingdefined from the N capacitors. The balancing system includes a pluralityof n balancing units for the n groups of capacitors, respectively. Eachbalancing unit includes a pair of associated switches connected throughan inter-switch junction and an electromagnetic coil. The inter-switchjunction is connected to an inter-capacitor junction of thecorresponding group of capacitors through the electromagnetic coil. Acontrol and generation circuit generates PWM control signals andtransmits a generated PWM control signal to each of the switches. Thecontrol and generation circuit is configured to generate the PWM controlsignals with fixed duty cycles that do not vary temporarily duringcharging or discharging of the N capacitors. The duty cycles of two ofthe PWM control signals to be transmitted to two associated switches arecomplementary for each balancing unit.

In the present disclosure, for each or some of the inter-capacitorjunctions (or nodes), a balancing circuit having a pair of connectedswitches is connected to the inter-capacitor junction through a coil,schematically in the form of a half H-bridge. The various capacitancevalues of the capacitors cause that the charges in the differentcapacitors increase differently. Consequently, the imbalance betweencapacitor voltages generates balancing currents through the coils, thatcontribute to balance the capacitor voltages. According to the presentdisclosure, the two switches are controlled by fixed duty cycles that donot vary temporarily during charging or discharging of the N capacitors.In other words, the two switches are turned on/off at fixed duty ratiosduring charging or discharging (that do not vary temporarily duringcharging or discharging).

The present disclosure also concerns a backup power supply systemincluding a stack of N capacitors consecutively connected and abalancing system as above defined for balancing the respective voltagesof the N capacitors during charging or discharging said N capacitors.

In an example embodiment, the stack of N capacitors consecutivelyconnected has two terminals and each pair of associated switches isconnected to said terminals of the stack of N capacitors.

In an example embodiment, the capacitors are supercapacitors.

In a particular embodiment, each pair of connected switches areconnected to the N capacitors.

In an example embodiment, the N capacitors having respective indices ifrom 1 to N, for each balancing unit having an inter-capacitor junctionbetween two capacitors of respective indices i and i+1 connected to theinter-switch junction, the control and generation circuit is configuredto generate a first PWM control signal having a duty cycle equal to(N−i)/N and a second PWM control signal having a duty cycle equal toi/N.

In another particular embodiment, for each balancing unit, the pair ofassociated switches is connected to two terminals of the correspondinggroup of capacitors.

In an example embodiment, the control and generation circuit isconfigured to generate PWM control signals that all have a duty cycle of50%.

Such a control and generation circuit is simpler.

In a particular embodiment, each group of capacitors includes only twocapacitors.

In an example embodiment, the control and generation circuit includes aflip-flop based frequency divider.

In an example embodiment, the control and generation circuit isconfigured to be connected to a power source and be provided with aclock signal from the said power source.

In an example embodiment, the control and generation circuit includes atleast one flip-flop and logical gates.

In an example embodiment, the control and generation circuit includes anumber of n flip-flops, wherein n is determined from the number N ofcapacitors according to the relation 2^(n-1)<N≤2^(n).

In an example embodiment, the system further comprises a power sourcethat is configured to:

produce a fixed charge current, during a first charging phase; and

during a second charging phase, after the voltage of the stack ofcapacitors consecutively connected has reached a predetermined targetvalue, produce a charge current modulated to maintain the voltage of theplurality of capacitors in series at the predetermined target value.

Said system may be integrated in a vehicle and supply power to anelectrical circuit of the vehicle and the power source includes anelectrical battery of the vehicle.

In an example embodiment, the N capacitors are arranged to supply powerto an electrical circuit of a vehicle when an electrical battery of thevehicle is temporary disconnected from the electrical circuit.

The present disclosure also concerns a vehicle comprising the backuppower supply system as previously defined, said backup power supplysystem being configured to be charged from a power source comprising anelectrical battery of the vehicle and to supply power to an electricalcircuit of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, purposes and advantages of the disclosure will becomemore explicit by means of reading the detailed statement of thenon-restrictive embodiments made with reference to the accompanyingdrawings.

FIG. 1 shows a schematic and simplified representation of a firstexemplary embodiment of the backup power supply system having twocapacitors and a pair of associated switches.

FIG. 2 shows schematically a second exemplary embodiment of the backuppower supply system, having four capacitors and three pairs ofassociated switches.

FIG. 3A shows simulation results of the capacitor voltages convergingwhen charging and balancing the backup power supply system of FIG. 2.

FIG. 3B shows PWM control signals transmitted to switches of the backuppower supply system in the simulation of FIG. 3A and the capacitorvoltages, after convergence (between 29.950 ms and 30 ms after start ofthe balancing operation).

FIG. 4 shows a third exemplary embodiment of the backup power supplysystem, having three capacitors and two pairs of associated switches.

FIG. 5 shows a fourth exemplary embodiment of the backup power supplysystem, having three capacitors and two pairs of associated switches,that is an alternative embodiment to embodiment of FIG. 4.

FIG. 6 shows a fifth exemplary embodiment of the backup power supplysystem, having N capacitors and N−1 pairs of associated switches, Nbeing an odd number;

FIG. 7 shows a sixth embodiment of the backup power supply system,having N capacitors and N−1 pairs of associated switches, N being aneven number.

FIG. 8 shows a seventh exemplary embodiment of the backup power supplysystem, having N capacitors and N−1 pairs of associated switches (only apart the N capacitors and N−1 pairs of switches being represented forthe sake of clarity), during charging of the capacitors from a powersource.

FIG. 9 shows the seventh exemplary embodiment of the backup power supplysystem of FIG. 8, during discharging of the capacitors to supply a load(or device).

FIG. 10 shows an exemplary embodiment of a circuit for generating PWMcontrol signals controlling the switches of the backup power supplysystem of FIG. 2.

FIG. 11 shows an exemplary embodiment of a power supply system of theprior art, having a power source connected in parallel to a plurality ofserially connected capacitors for buffering energy, supplying a load P.

FIG. 12 shows a first exemplary embodiment of a balancing circuit forcapacitors of a power supply system, of the prior art.

FIG. 13 shows a second exemplary embodiment of a balancing circuit forcapacitors of a power supply system, of the prior art, that is animproved variant of the first exemplary embodiment of FIG. 12.

FIG. 14 shows a third exemplary embodiment of an improved balancingcircuit for capacitors of a power supply system, of the prior art.

DETAILED DESCRIPTION

Beforehand, it is stated that the same, analogous or correspondingelements represented on different figures have the same reference,unless otherwise stated.

The present disclosure concerns a backup power supply system 100 thatcan be used, for example in a vehicle, as an energy reserve available tobe used when it is needed, for example when a battery power supply isinterrupted due to a disconnection of the battery. The backup powersupply system 100 is charged from a power source 200, for example anelectrical battery (typically a 12V battery) in a vehicle. When thebattery power supply is interrupted, the backup power supply isdischarged in order to supply current to one or more loads, that areelectrical devices of the vehicle.

The backup power supply system 100 has

-   -   a set or stack 10 of N capacitors C1, C2, . . . , Ci, . . . , CN        that are serially connected and    -   a balancing circuit 20 for balancing the respective voltages of        the N capacitors during charging or discharging of the N        capacitors.

In an example embodiment, the capacitors C1, C2, . . . , Ci, . . . , CNare supercapacitors.

A number of n groups of at least two consecutive capacitors, with n≤N−1,are defined from the N capacitors 10. Conventionally, an index jdesignates the index of a group of at least two capacitors with 1≤j≤n inthe present description.

For each group of capacitors of index j, the balancing circuit 20 has apair of associated and serially connected switches. The pairs ofassociated switches are referenced as SWj1 and SWj2 with 1≤j≤n (namelySW11, SW12, . . . , SWj1, SWj2, . . . , SWn1, SWn2) in the figures. Aninter-switch junction, or node, related to each pair of associatedswitches (that is the junction or node between the two switches) isconnected to an inter-capacitor junction, or node, related to said groupof capacitors (that is a junction or node between two capacitors of thesame group) through an electromagnetic coil L1, . . . , Lj, . . . , Ln.The group including:

one group of index j having at least two capacitors,

the corresponding pair of associated switches SWj1 and SWj2 and

the corresponding electromagnetic coil Lj form a balancing unit namedBUj in the description.

The backup power supply system 100 also includes a control andgeneration circuit 30 for generating PWM (Pulse Width Modulation)control signals PWMj1, PWMj2 and transmitting a generated PWM controlsignal PWMj1, PWMj2 to each of the switches SWj1, SWj2, with 1≤j≤n, inorder to control the switches. In the present disclosure, the controland generation circuit generates PWM control signals PWMj1, PWMj2 havingfixed duty cycles.

The expression “fixed duty cycles” means that the respective duty cyclesdo not vary temporarily during charging or discharging of the Ncapacitors. In other words, the duty cycles of the control signals usedto turn on/off the switches are constant over time during charging ordischarging.

For each balancing unit BUj, the two generated PWM control signal PWMj1,PWMj2 are complementary (PWMj1=PWMj2 ), which means that the firstsignal PWMj1 is ON (or high level) when the second signal PWMj2 is OFF(or low level). So, the sum of the fixed duty cycles DC of the two PWMcontrol signals PWMj1 and PWMj2 (respectively transmitted to twoassociated switches SWj1 and SWj2 with 1≤j≤n) is equal to 100%.

The backup power supply system 100 operates the switches according to aswitching frequency f_(sw). For example, the backup power supply system100 uses a clock signal from the power source 200 (for example a powerDCDC converter) as a basis frequency f_(b) to control the switches. Fora stack of N capacitors, the switching frequency f_(sw) is equal to thebasis frequency f_(b) divided by N: f_(sw)=f_(b)/N. This configurationprovides the advantage that there is no need to generate a frequency. Asa result, the amount of resources or components can be reduced.Alternatively, an external basis frequency generator can be used. Inthat case, the frequency is advantageously determined according to adesired switching frequency for balancing the capacitors:f_(b)=f_(sw)·N.

When the capacitors are being charged, they are connected to a powersource 200. More precisely, the two terminals of the stack of N seriallyconnected capacitors are connected to the two terminals of the powersource 200. Conventionally, the capacitor of index 1 is connected to thepositive terminal of the power source 200 and the capacitor of index Nis connected to the negative terminal of the power source 200 (forexample connected to the ground). The references u_total and i_chargedesignate the voltage and the electrical current provided by the powersource 200. When the capacitors are used to supply one or more loads,the connection with the power source 200 is interrupted and the stack ofN serially connected capacitors is connected to the load(s).

During charging (or discharging) of the N stacked capacitors, for eachbalancing unit BUj balancing a group of capacitors, an imbalance betweenthe respective capacitor voltages of the different capacitors within thegroup causes an electrical current i_bal through the coil Lj. Thiselectrical current i_bal allows to balance the capacitor voltages. Thebasic principle is that the capacitor having the highest voltagedischarges into the capacitor having the smallest voltage (in case thegroup has two capacitors).

We first assume that the N serially connected capacitors are grouped bytwo and two adjacent groups of two capacitors have one middle capacitorin common. It means that N−1 groups of two consecutive connectors aredefined.

FIG. 1 shows a schematic representation of the backup power supplysystem 100 according to a first exemplary embodiment that is verysimplified. In this simplified embodiment, the backup power supplysystem 100 has only one balancing unit BU. More precisely, the backuppower supply system 100 has only two capacitors C1, C2 and a pair of twoassociated switches SW1, SW2, connected to one another through ainterswitch junction. The two switches SW1 and SW2 are connected “inparallel” to the two capacitors C1, C2 (i.e., the two terminals of thepair of switches SW1, SW2 are respectively connected to the twoterminals of the stack of capacitors C1, C2), but the inter-capacitorjunction between the two capacitors C1 and C2 and the inter-switchjunction between the two switches SW1, SW2 are connected through a coilL. The backup power supply system 100 has also a control and generationcircuit 30 that generates two PWM control signals PWM1 and PWM2 andtransmits them to the two switches SW1 and SW2, respectively, in orderto control them. The respective duty cycles DC of these two PWM controlsignals PWM1 and PWM2 are fixed and both equal to 50%. In order tocharge the two capacitors C1 and C2, the balancing unit BU is connectedto a power source 200, as represented in FIG. 1 (the BU is connected tothe terminals of the power source 200). The control and generationcircuit 30 can also be connected to the power source 200 and supplied byit.

During the operation of charging the capacitors C1, C2 through thebalancing circuit 20, the capacitor voltages converge to the same valueof u_total/2, u_total being the voltage of the power source 200, withina short period of time, inferior to one second, as will be explained inmore detail later in the description. In FIG. 1, the reference i_bal12represents the current through the coil when the top capacitor C1discharges into the bottom capacitor C2, and the reference i_bal21represents the current through the coil when the top capacitor C2discharges into the bottom capacitor C1.

FIG. 2 shows a schematic representation of the backup power supplysystem 100 according to a second exemplary embodiment.

In the second exemplary embodiment, the backup power supply system 100has a stack of four capacitors C1, C2, C3 and C4 (N=4), serially (i.e.,consecutively) connected, and three pairs of associated switchesSW11-SW12, SW21-SW22, SW31-SW32. Each pair of switches SW11-SW12(SW21-SW22 and SW31-SW32) is connected ‘in parallel’ to the stack ofcapacitors C1-C4 (i.e., the two terminals of each pair of switchesSW11-SW12, SW21-SW22, SW31-SW32 are respectively connected to the twoterminals of the stack of capacitors C1-C4). The three inter-capacitorjunctions are connected to the three inter-switch junctions of the threepairs of switches SW11-SW12, SW21-SW22, SW31-SW32 through three coilsL1, L2 and L3, respectively.

A control and generation circuit (not represented in FIG. 2) generatesPWM control signals with fixed duty cycles to control the switchesSW11-SW12, SW21-SW22, SW31-SW32. The values of the duty cycles forcontrolling SW11-SW12, SW21-SW22, SW31-SW32 are as follows:

DC11=75%, DC12=25%

DC21=50%, DC22=50%

DC31=25%, DC32=75%

so as to set the voltages of the junction nodes between C1 and C2,between C2 and C3 and between C3 and C4 to 75%. u_total, 50%. u_totaland 25%. u_total, respectively, after balancing.

FIG. 3A shows a simulation result of the evolution over time of thecapacitor voltages u_C1, u_C2, u_C3, u_C4 of the four capacitors C1, C2,C3 and C4 when these capacitors are charged from the power source 200,through the balancing circuit 20. For the simulation, the voltageu_total of the power source 200 is for example set to 7.2V.Consequently, a target value of the capacitor voltage for each of thecapacitors C1, C2, C3 and C4 is equal to 1.8V (7.2V divided by 4).

Initially, the capacitor voltages u_C1, u_C2, u_C3, u_C4 are for exampleequal or close to zero. The time evolutions of the different capacitorvoltages u_C1, u_C2, u_C3, u_C4 are analogous, but not identical as thecapacitors have different capacitance values.

The charging operation has two main phases: a first quick phase I and asecond slow phase II. The first quick charging phase I has a first (orinitial) part T1 during which there is no balancing. The capacitorvoltages u_C1, u_C2, u_C3, u_C4 start increasing but diverge, whichmeans that they become more and more different from one another, due tothe lack of balancing. This phenomenon is due to the fact that thebalancing circuit 20 needs that a certain amount of voltage be reached,for example a voltage threshold of around 3 or 4 V at the terminals ofthe stack of the N serial capacitors, in order for the switches to startworking. However, depending on the technology used for the switches, thelatter may work before reaching this N-capacitor voltage threshold.

As soon as the balancing starts, the capacitor voltages u_C1, u_C2,u_C3, u_C4 converge, which means that they come closer to each other.This corresponds to a second and subsequent part T2 of the quickcharging phase I. Then, as soon as the maximum total voltage u_totalacross the capacitor stack is reached (7.2V in the present example), theslow phase II starts. During the slow phase II, the capacitor voltagesu_C1, u_C2, u_C3, u_C4 across the different capacitors increase moreslowly or decrease a little and converge to a common voltage. As canalso be seen in FIG. 3A, the capacitor voltages u_C1, u_C2, u_C3, u_C4of all capacitors C1 to C4 reach and keep a value equal or very close tothe target value of 1.8V after a short time period of around 15 ms.

In the exemplary embodiment of FIG. 3A, during the first phase I ofcharging, the power source 200 produces a fixed charge current. Duringthe second phase II of charging, after the voltage of the N stackedcapacitors has reached the predetermined target value (u_total), thepower source 200 produces a charge current that is modulated orregulated in order to maintain the voltage of the N stacked capacitorsat the predetermined target value (u_total).

FIG. 3B shows the PWM control signals PWM11, PWM21 and PWM31,transmitted to switches SW11, SW21 and SW31 respectively, and the timeevolutions of the capacitor voltages u_C1, u_C2, u_C3, u_C4, during atime period between 29,950 ms and 30 ms (from the beginning of thecharging operation). FIG. 3B shows that, after a period of time ofaround 30 ms from the start of the charging operation, all capacitorvoltages u_C1, u_C2, u_C3, u_C4 have reached and maintain voltage valueequal to the capacitor voltage target value (1.8V) with an accuracy ofaround 2 or 3%.

The balancing circuit 20 allows that each of the N stacked capacitorsreaches the target voltage value in a fast, accurate and efficientmanner.

FIG. 4 shows a third exemplary embodiment that only differs from thesecond exemplary embodiment in that the stack of capacitors has onlythree serially connected capacitors C1, C2, C3 and two pairs ofassociated switches SW11-SW12 and SW21-SW22. The inter-switch junctionsare connected to the two inter-capacitor junctions through two coils L1,L2, respectively.

In some exemplary embodiments (such as the second and third exemplaryembodiments), each pair of associated switches is connected ‘inparallel’ to a stack of N serially connected capacitors (i.e., the twoterminals of each pair of switches are connected to the two terminals ofthe N stacked capacitors, respectively). For each balancing unit BUj,the inter-switch junction between SWj1 and SWj2 is connected to aninter-capacitor junction between two capacitors of respective indices iand i+1 of the same group j through the coil Lj, and the control andgeneration circuit 20 generates a first PWMj1 control signal having aduty cycle equal to (N−i)/N and a second PWMj2 control signal having aduty cycle equal to i/N.

In other exemplary embodiments (that are a variant of the aboveembodiments), for each balancing unit BUj associated with the group j ofat least two capacitors, the pair of associated switches SWj1-SWj2 isconnected in parallel to the group j of stacked capacitors. When ngroups of at least two consecutive capacitors are defined from the Ncapacitors, n pairs of associated switches are connected ‘in parallel’the n groups of capacitors, respectively. In addition, for eachbalancing unit BUj associated with the group j of at least twocapacitors, the inter-switch junction between SWj1 and SWj2 is connectedto an inter-capacitor junction between two capacitors of respectiveindices i and i+1 of the group j through a coil Lj. In these variantembodiments, the control and generation circuit generates PWM controlsignals that all have a fixed duty cycle of 50% for all the switchesSWj1-SWj2 with 1≤j≤n.

FIG. 5 is a fourth exemplary embodiment, according to the abovedisclosed variant. More precisely, the embodiment of FIG. 5 is a variantof the embodiment of FIG. 4 according to the above paragraph. In thisFIG. 5, the stack of capacitors has three serially connected capacitorsC1, C2, C3 and two pairs of associated switches SW11-SW12 and SW21-SW22balances the capacitor voltages. More precisely, the pair of switchesSW11-SW12 is connected ‘in parallel’ to the two capacitors C1, C2 (i.e.,the two terminals of the pair of switches SW11-SW12 are connected to thetwo terminals of the capacitors C1-C2) and the pair of switchesSW21-SW22 is connected ‘in parallel’ to the two capacitors C2, C3 (i.e.,the two terminals of the pair of switches SW21-SW22 are connected to thetwo terminals of the capacitors C2-C3). The inter-switch junctions areconnected to the inter-capacitor junctions through the coils L1 and L2.The duty cycles of all PWM control signals transmitted to switchesSW11-SW12 and SW21-SW22 are fixed and equal to 50%.

FIG. 6 shows a fifth exemplary embodiment that is a generalization ofthe fourth embodiment of FIG. 5, with a number N of serially connected(stacked) capacitors, N being an odd number. For each balancing unitBUj, the pair of associated switches SWj1-SWj2 is connected ‘inparallel’ to the corresponding group of two capacitors (i.e., the twoterminals of switches SWj1-SWj2 are connected to the two terminals ofthe corresponding group of two capacitors) and the inter-switch junctionis connected to the inter-capacitor junction through a coil Lj. Thereare N−1 coils that connect each of the N−1 inter-capacitor junctions toN−1 inter-switch junctions belonging to the N−1 pairs of associatedswitches SWj1-SWj2 with 1≤j≤N−1, respectively.

FIG. 7 shows a sixth embodiment that only differs from the sixthembodiment in that the number N of serially connected (stacked)capacitors is an even number.

FIG. 8 shows a seventh exemplary embodiment that is a generalization ofthe embodiments of FIGS. 2 and 4, with a number N of serially connected(stacked) capacitors. The N serially connected capacitors haverespective indices i from 1 to N. The capacitor C1 is connected to thepositive terminal of the power source 200 and the capacitor CN isconnected to the negative terminal of the power source 200 (here to theground). For each balancing unit BUj, the pair of associated switchesSWj1-SWj2 is connected ‘in parallel’ to the N serially connected(stacked) capacitors (i.e., the two terminals of the pair of associatedswitches SWj1-SWj2 are connected to the two terminals of the N stackedcapacitors) and the inter-switch junction between SWj1 and SWj2 isconnected to an inter-capacitor junction through a coil Lj. When theinter-capacitor junction between capacitors Ci and Ci+1 are connected tothe inter-switch junction between SWj1 and SWj2, a first PWM controlsignal having a duty cycle equal to (N−i)/N is generated and transmittedto a first of the two associated switches SWj1 and a second PWM controlsignal having a duty cycle equal to i/N is transmitted to the second ofthe two associated switches SWj2. The first switch SWj1 is connected tothe positive terminal of the power source 200 and the second switch SWj2is connected to the negative terminal of the power source 200 (hereconnected to the ground).

FIG. 9 shows the backup power supply system according to the seventhembodiment of FIG. 8, during discharging of the N capacitors in order tosupply a load P. The connection of the stack of N capacitors to thepower source is interrupted and the capacitors discharge into the load.The balancing circuit allows to balance the respective voltages of the Ncapacitors during discharging of the N capacitors.

In an eighth exemplary embodiment of the backup power supply system, ngroups of two or more capacitors are defined from the N capacitors andat least part of the n groups have more than two capacitors. Suchembodiment is appropriate when the number N of capacitors is important.

According to the present disclosure, the control and generation circuitgenerates PWM control signals having fixed duty cycles, which allows tobalance quickly the capacitor voltages of the N serially connectedcapacitors without feedback loop.

In some exemplary embodiments, each pair of associated switches areconnected ‘in parallel’ to the N serially connected capacitors. In otherexemplary embodiments, j pairs of associated switches are connected ‘inparallel’ to j groups of at least two capacitors.

The generation of these PWM control signals having fixed duty cycles canbe achieved using flip-flops and logical gates. The control andgeneration circuit uses a clock signal that is transmitted from thepower source. The flip-flops operate as frequency dividers. A number ofx flip-flops allows to generate the duty-cycles of a maximum of 2^(x)capacitors connected in series. More generally, the control andgeneration circuit 30 has a number of n flip-flops, wherein n isdetermined from the number N of capacitors according to the relation2^(n-1)<N≤2^(n).

FIG. 11 shows an exemplary embodiment of a circuit for generating threePWM control signals having respective duty cycles of 25%, 50% and 75%.Such a circuit is well-known by the skilled person and will not bedescribed in more detail.

The generation of PWM control signals having one common duty cycle of50% allows to use a control and generation circuit that is simple, hassmaller components and produces less current ripple.

The reduction of the current ripple is noticeable in particular when thebasis frequency, used to control the switches and balance thecapacitors, is provided by the power source 200. For example, for astack of four capacitors C1 to C4, a plurality of PWM control signalshaving respective duty-cycles of 25%, 50% and 75% are generated, whichimplies to divide the basis frequency by four in order to obtain theswitching (or balancing) frequency. The generation of PWM controlsignals produces a higher current ripple for a lower frequency. Thegeneration of PWM control signals having only a 50% duty-cycle onlyneeds to divide the basis frequency by two. Therefore, through thehigher switching frequency (basis frequency divided by two) of thebalancing circuit, less current ripple is produced. When an externalbasis frequency generator is used and its frequency is determinedaccording to the desired switching frequency for balancing thecapacitors, there is no noticeable difference of current ripple.

As previously indicated, the backup power supply system 100 is forexample integrated in a vehicle and supplies power to an electricalcircuit of the vehicle. In such a case, the power source 200 is anelectrical battery of the vehicle, such as a 12V battery. The N seriallyconnected capacitors supply power to the electrical circuit of thevehicle when the electrical battery of the vehicle is temporarydisconnected from the electrical circuit.

The present disclosure also concerns the balancing circuit 20 forbalancing respective voltages of N serially connected capacitors of abackup power supply system, during charging or discharging of said Ncapacitors, n groups of at least two consecutive capacitors, with n≤N−1,being defined from the N capacitors, as previously described.

In particular, the balancing circuit 20 has, for each group ofcapacitors, a pair of associated and connected switches with aninter-switch junction that is connected to an inter-capacitor junctionof said group of capacitors through an electromagnetic coil, said groupof capacitors, said pair of associated switches and said electromagneticcoil forming a balancing unit; and a control and generation circuit forgenerating PWM control signals and transmitting a generated PWM controlsignal to each of the switches, that generates PWM control signalshaving fixed duty cycles.

The duty cycles of two PWM control signals transmitted to two associatedswitches are complementary for each balancing unit.

The present disclosure also concerns a vehicle integrating the backuppower supply system and/or the balancing circuit previously described.

The preceding description is illustrative rather than limiting innature. Variations and modifications to the disclosed exampleembodiments may become apparent to those skilled in the art that do notnecessarily depart from the essence of the invention. The scope of legalprotection given to the invention can only be determined based on thefollowing claims.

We claim:
 1. A balancing system for balancing respective voltages of Nconsecutively connected capacitors of a backup power supply systemduring charging or discharging of said N capacitors, n groups of atleast two consecutive capacitors, with n≤N−1, being defined from the Ncapacitors, said balancing system comprising: a plurality of n balancingunits for the n groups of capacitors, respectively, each balancing unitcomprising a pair of associated switches connected through aninter-switch junction and an electromagnetic coil, said inter-switchjunction being connected to an inter-capacitor junction of thecorresponding group of capacitors through said electromagnetic coil; anda control and generation circuit for generating PWM control signals andtransmitting a generated PWM control signal to each of the switches;wherein the control and generation circuit is configured to generate PWMcontrol signals having fixed duty cycles, that do not vary temporarilyduring charging or discharging of the N capacitors, the duty cycles oftwo PWM control signals to be transmitted to two associated switchesbeing complementary for each balancing unit.
 2. A backup power supplysystem including a stack of N capacitors consecutively connected and thebalancing system according to claim 1 for balancing the respectivevoltages of the N capacitors during charging or discharging said Ncapacitors.
 3. The backup power supply system according to claim 2,wherein the stack of N capacitors has two terminals and each pair ofassociated switches is connected to said terminals of the stack of Ncapacitors.
 4. The backup power supply system according to claim 3,wherein the N capacitors have respective indices i, i is from 1 to N,and for each balancing unit having an inter-capacitor junction betweentwo capacitors of respective indices i and i+1 connected to theinter-switch junction, the control and generation circuit is configuredto generate a first PWM control signal having a duty cycle equal to(N−i)/N and a second PWM control signal having a duty cycle equal toi/N.
 5. The backup power supply system according to claim 2, wherein,for each balancing unit, the pair of associated switches is connected totwo terminals of the corresponding group of capacitors.
 6. The backuppower supply system according to claim 5, wherein the control andgeneration circuit is configured to generate PWM control signals thatall have a duty cycle of 50%.
 7. The backup power supply systemaccording to claim 2, wherein each group of capacitors includes only twocapacitors.
 8. The backup power supply system according to claim 2,wherein the control and generation circuit includes a flip-flop basedfrequency divider.
 9. The backup power supply system according to claim8, wherein the control and generation circuit is configured to beconnected to a power source and be provided with a clock signal fromsaid power source.
 10. The backup power supply system according to claim2, wherein the control and generation circuit includes at least oneflip-flop and logical gates.
 11. The backup power supply systemaccording to claim 10, wherein the control and generation circuitincludes a number of n flip-flops, and n is determined from the number Nof capacitors according to the relation 2^(n-1)<N≤2^(n).
 12. The backuppower supply system according to claim 2, comprising a power source thatis configured to: produce a fixed charge current, during a firstcharging phase; and during a second charging phase, after the voltage ofthe stack of N capacitors has reached a predetermined target value,produce a charge current modulated to maintain the voltage of the stackof consecutively connected capacitors at the predetermined target value.13. The backup power supply system according to claim 2, wherein thestack of N capacitors are configured to supply power to an electricalcircuit of a vehicle when an electrical battery of the vehicle istemporarily disconnected from the electrical circuit.
 14. A vehiclecomprising the backup power supply system according to claim 2, whereinthe backup power supply system is configured to be charged from a powersource comprising an electrical battery of the vehicle and to supplypower to an electrical circuit of the vehicle.